KY Jon;
I agree with all that & I don't lengthen my cones. However I still contend that the average cone job does not constitute a hazard. With the exception of those 4" cones & back bored jobs, which generally aren't done on one of the classic doubles we are talking about here, the juncture of the chamber to cone is still the thinnest point in the entire chamber/cone area. I didn't set up & measure wall thickness but did measure barrel OD on the lightest weight 12ga double I own. This is a LAC/Ithaca H grade with twist barrels & 2 3/4" chamber. "IF" concentric the thickness over the juncture point is .095". Again "IF" the cones were lengthened to 1½" the wall at 4¼" would still be .095". If that thickness of wall will hold the pressure at 2 3/4" it will certainly hold it at 4¼".
If you loaded both a .410 & A 12 gauge with 3/4 oz of shot the .410 would have about 78% more shot in bore contact. I would not expect lengthening the cones to provide the same benefit to the 12 as to the .410.
The only point I am arguing is the metal in the cone is in about 99.98% of the time not the weakest link in the chain. It would be truly an exceptional case for it to be so.

Miller/TN
I Didn't Say Everything I Said, Yogi Berra